The back mount involves you being behind your opponent with your legs hooked around them.
This is an excellent position to apply chokes since your opponent can't see what you're doing.
This is the back mount used when your opponent is on all-fours.
Be sure to hook your feet around your opponent so they can't throw you off.
The back mount can also be used when you're on your back. If your opponent tries to roll you off from the first back mount you may find yourself in this position.
Again, remember to hook your feet on your opponents hips. NEVER cross your feet, otherwise you will be open to a nasty foot lock.